1. Field of the Invention
The present invention relates to a method for producing friction materials to be used in brakes, clutches, and the like, for cars, railway cars and the like.
2. Description of the Related Art
General method for producing friction materials for brakes or clutches such as brake pads, brake linings, clutch facings or the like includes a dry method and wet method. The dry method has processes of uniformly mixing a thermosetting resin binder such as phenolic resin, etc., fibrous reinforcing material such as glass fibers and aramid fibers, friction modifier including rubber dust and cashew dust, and metal particles, and subsequently heating and press-forming the raw mixture material. More specifically, in the dry method the raw mixture material is preformed in a mold, and the preform is transferred to another mold where a backing plate is set, and the preform is heated and pressurized to unite with the backing plate, and then cured thereafter. The wet method, on the other hand, includes processes of mixing and wetting particles of all raw materials with solvent, drying the wet mixture of raw materials, and heating-and-press forming the material.
These dry and wet methods suffer from problems in work environment. That is, coarse particulates are generated during the production of the friction material in the dry method, and the wet method has been utilizing organic solvent which is not good for human health such as toluene, acetone, methyl ethyl ketone or the like. Further, the dry method has a difficulty in producing friction materials having a stable characteristics because some components of the fibrous reinforcing materials or the friction modifier materials or the like may cause the dispersion defect during the mixture of the raw materials. In case of the wet method, on the other hand, the resin materials and the like are completely dissolved with the solvent in the mixture of raw materials and the wettability with the other materials is good so that the density and strength of the materials are improved. However, since the amount of blow holes decreases the damping characteristic which is required as a friction material is lowered and, as a result, the frictionability of the material by the wet method Would be worse than that by the dry method.
In order to avoid the afore-mentioned difficulties it has been proposed another wet method utilizing water or latex as a wet agent which do not dissolve resin materials or the like. Even by this method, however, it is difficult to prevent the undesirable coarse particulates from occurring because the granules may move while drying or granulating the materials or may be broken due to vibration. This is because that the viscosity force of the granulated material is low in this method even the wet raw mixture is previously granulated and dried for avoiding the coarse particulates during the drying process.